It is known to directly reduce iron ore by use of a reducing gas formed through the gasification of hydrocarbon. Natural gas is conventionally converted thermally or catalytically, if necessary with the addition of steam and/or oxygen, to a reducing gas formed principally of carbon monoxide and hydrogen, with the ratio between these two components being 1:1 or less. This reducing gas is then fed substantially at the temperature it attains in the conversion process into the shaft furnace where the direction reduction takes place. A portion of the stack gas produced by direct reduction is fed back to the converter as an oxygen-carrier where its carbon monoxide and hydrogen content is exploited. Another portion of the stack gas is used for simple heat exchange.
The principal difficulty with such a system is that it must employ relatively expensive basic fuels for the production of the original reducing gas. Thus natural gas or other light hydrocarbons must be employed.
It is known to gasify coal with oxygen to produce a mixture of carbon monoxide and hydrogen. See for example "Vergasung fester Brennstoffe und Oxydative Unwandlung von Kohlenwasserstoffen" by Jacques Meunier (Verlag Chemie GmbH, Weinheim/Bergstrasse: 1962 ).
The synthetic-gas mixture so produced has a carbon monoxide/hydrogen ratio of at least 1.5:1. Use of this gas as a reduction gas produces a very wet stack gas having a carbon monoxide or carbon dioxide to hydrogen and water ratio of approximately 1.5:1. Filtering out of the water and dust in this stack gas by heating it up again to 1000.degree. C. causes the carbon monoxide to decay into soot and carbon dioxide, making the operation extremely messy and, therefore, costly.